For all the Deposit and Withdrawal transactions, in the transaction detail page there is a button which can be turned on or off called Use the market conversion rate:
If the button is turned on, CryptoBooks will use the transaction date and time to automatically calculate the buying or selling price of the bought or sold token by retrieving the value from the market.
If the button is turned off, the user will have to manually digit the price of the token that has been bought or sold.
When creating a manual transaction the button is visible and usable by default, while
in case of transactions already created or imported (also automatically) press the Edit button in order to see it.
!!! IMPORTANT !!!
Trade transactions do not have this button because the conversion rate (buying and selling prices) is determined by the amount of the two coins traded in the transaction.